Method of making air/fuel ratio sensor

ABSTRACT

An air/fuel ratio sensor is described, comprising an oxygen concentration electrochemical cell and an oxygen pump disposed in a face-to-face relationship with a gap being formed therebetween, the side of said electrochemical cell opposite the side facing said oxygen pump being in contact with the atmosphere, said gap forming a gas diffusion compartment that communicates with a gas to be analyzed by a gas diffusion limiting means, wherein the gap existing between said oxygen concentration electrochemical cell and said oxygen pump has a width of no more than 0.2 mm and no less than 0.01 mm.

This is a division of application Ser. No. 07/228,808, filed Jul. 29,1988, which is a continuation of application Ser. No. 06/832,800, filedFeb. 25, 1986, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to an air/fuel (A/F) ratio sensor fordetecting the A/F ratio of an air/fuel mixture being supplied into acombustor. More particularly, the present invention relates to an A/Fratio sensor that is capable of detecting the A/F ratio of an air/fuelmixture using an oxygen ion conductive solid electrolyte over the fulloperating range, including the lean region (where air is in excess ofthe stoichiometric value) to the rich region (where fuel is in excess ofthe stoichiometric value).

With a view to improving fuel economy and reducing emissions, someconventional combustors such as internal combustion engines, have beenprovided with the capability of feedback control, involving thedetection of oxygen levels in the exhaust and control of the air/fuelmixture in the combustion chamber so as to burn it at an A/F ratio inthe vicinity of the stoichiometric value. An oxygen sensor commonly usedto detect the concentration of oxygen in the exhaust employs an ionconductive solid electrolyte with coatings of porous electrode layersand detects the burning of fuel at an A/F ratio in the vicinity of thestoichiometric value, depending upon the change in the electromotiveforce generated by the difference between the oxygen partial pressure ofthe exhaust and that of air. Generally, this type of oxygen sensorproduces an output voltage that changes abruptly at the stoichiometricA/F ratio of the air/fuel mixture.

Attempts are being made to maximize the performance of combustors inaddition to fuel economy improvements and emissions reduction by meansof performing feedback control to attain a desired A/F ratio that isadaptive to a specific state of operation of the combustor. This goal,however, is not attained by the aforementioned oxygen sensor, which ismerely capable of detecting the stoichiometric A/F ratio of the air/fuelmixture.

A sensor or analyzer capable of performing the above described A/F ratiofeedback control has recently been proposed in Unexamined PublishedJapanese Patent Application Nos. 72286/1977 and 66292/1978; this deviceis provided with a chamber that forms a closed space including thesurface of one of the two electrodes formed on a solid electrolyte and asmall diffusion aperture is formed in the wall of this chamber; avoltage is applied across the two electrodes so that a gas component inthe gas to be analyzed will be introduced into the chamber by diffusion;and the amount of current flowing through said solid electrolyte ismeasured to determine the concentration of the particular gas component.

In the device described above, the atmosphere around one of the twoelectrodes formed on a solid electrolyte provides a closed space thatcommunicates with the atmosphere of the gas to be analyzed by means of asmall diffusion limiting aperture. One major problem with this device isthat the diffusion limiting means is difficult to fabricate since thediffusion limited current value must be measured in order to determinethe concentration of a particular gas being monitored in the gas to beanalyzed.

SUMMARY OF THE INVENTION

In order to solve the aforementioned problem, the present inventionprovides a novel A/F ratio sensor comprising an oxygen concentrationelectrochemical cell and an oxygen pump disposed in a face-to-facerelationship with a gap being formed therebetween, the side of saidelectrochemical cell opposite the side facing said oxygen pump being incontact with the atmosphere, said gap forming a gas diffusioncompartment that communicates with a gas of interest by way of a gaslimiting means, wherein the gap existing between said oxygenconcentration electrochemical cell and said oxygen pump has a width ofno more than 0.2 mm and no less than 0.01 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial fragmentary perspective view of the A/F ratio sensorin accordance with one embodiment of the present invention;

FIG. 2 is an exploded view of that sensor; and

FIGS. 3 and 4 show the operating characteristics of the same sensor.

FIGS. 5 and 6 are illustrations similar to FIGS. 1 and 2, respectively,but showing the use of particles to prevent deformation of the diffusioncompartment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Each of the oxygen concentration electrochemical cell and the oxygenpump is formed on a solid electrolyte plate, such as a solid solution ofY₂ O₃ -ZrO₂, provided with a porous electrode on both sides thereof.

A typical example of the material of the solid electrolyte plate is asolid solution of zirconia and yttria or calcia. Other usable materialsinclude solid solutions of cerium dioxide, thorium dioxide, and hafniumdioxide, a solid solution of a perovskite type oxide, and a solidsolution of a trivalent metal oxide.

The porous electrode may be formed from platinum or gold by variousmethods; in one method, a powder of a suitable material selected fromthe above listed metals that is used as the principal component isformed into a paste, and the paste is printed in a predetermined patternon the solid electrolyte by a thick-film deposition technique, followedby sintering of the printed coat; in another method, the powder of thestarting material is applied onto the solid electrclyte by a suitablethin-film depositing technique such as flame spraying, chemical platingor evaporation.

If desired, two solid electrolyte plates may be provided; an oxygenconcentration electrochemical cell and an oxygen pump are formed onopposite sides of one solid electrolyte plate, and another oxygen pumpis formed on the other solid electrolyte plate. This arrangement isadvantageous in that the oxygen pump will have an improved ability tolet oxygen gas in and out of the diffusion compartment described laterin this specification, thereby providing easier control of the oxygenpartial pressure in the vicinity of the electrodes on the oxygenconcentration electrochemical cell. It is, however, preferable for thepurpose of the present invention that the greater part of one majorsurface of the diffusion compartment should be assumed by an electrodeon the oxygen pump. It should also be noted that the area of theelectrodes on the oxygen pump is preferred to be no smaller than 5 mm².

An atmosphere-introducing channel should be provided by a known methodon at least the surface of the oxygen concentration electrochemical cellthat does not face the diffusion compartment. This channel may beprovided by joining a channel former composed of a U-shaped stressrelaxing layer and a tabular support to the surface of a solidelectrolyte that does not face the diffusion compartment.

The gas diffusion limiting means may be provided by one or moreapertures that establish communication between the diffusion compartmentand the atmosphere of the gas to be analyzed. Such apertures arepreferably filled with a porous material so as to provide an increasedresistance to gas diffusion.

The diffusion compartment is formed by joining a solid electrolyte platefor the oxygen concentration electrochemical cell to another solidelectrolyte plate for the oxygen pump, with a spacer having a cavitythat forms part of the diffusion compartment being interposed betweenthe two solid electrolyte plates. Prior to sintering, a single layer ofgranulated particles prepared from a spray dryer having a diameterapproximately equal to the width of the diffusion compartment as shownin the illustrations of FIGS. 5 and 6 wherein the particles aredesignated by reference numerals 220 may be placed in said compartment,and this is preferred for the purpose of preventing the diffusioncompartment from deforming during the firing step.

The thickness of the diffusion compartment, or the distance between theopposing electrodes on the oxygen concentration electrochemical cell andthe oxygen pump, is in the range of 0.01 to 0.2 mm, with the upper limitof 0.1 mm being preferred. If the thickness of the diffusion compartmentis smaller than 0.01 mm, the compartment will limit the diffusion ofoxygen gas so much as to decrease the response of the A/F ratio sensor.In addition, sensors of uniform quality cannot be fabricated since thethin diffusion compartment will easily deform during manufacture andpresents considerable difficulty in ensuring the desired electricalinsulation. If, on the other hand, the thickness of the diffusioncompartment exceeds 0.2 mm, the differential pressure of the gas to beanalyzed will change significantly within the diffusion compartment, andin particular, the differential pressure that exists between theopposing electrodes on the oxygen concentration electrochemical cell andthe oxygen pump will be increased to cause not only an unduly greatincrease in the pumping current but also degraded response of thesensor. The increase in the differential pressure of the gas of interestthat results from the use of an excessively thick diffusion compartmenthas been found to cause a problem even if the oxygen concentrationelectrochemical cell is designed to produce an output voltage ofapproximately 500 mV, typically 450 to 500 mV, when the sensor isoperating for measurement purposes.

The operation of the A/F ratio sensor of the present invention describedabove will proceed as follows.

When the air/fuel mixture is in the lean region, the sensor is put intothe exhaust gas and the electrode on the side of the oxygen pump whichfaces the atmosphere is supplied with a positive voltage while anegative voltage is applied to the electrode on the side facing thediffusion compartment. As a result, oxygen ions will move through thesolid electrolyte of the oxygen pump toward the side opposite thediffusion compartment, whereby the oxygen gas in the diffusioncompartment is pumped out of said compartment.

As the oxygen gas in the diffusion compartment is pumped out in themanner described above, a difference is produced between theconcentration of oxygen on the side of the oxygen concentrationelectrochemical cell facing the atmosphere and the concentration ofoxygen within the diffusion compartment, because of the oxygen diffusionlimiting action of the diffusion limiting section. This differentialoxygen concentration enables the oxygen concentration electrochemicalcell to produce an electromotive force. If the amount of current flowingthrough the oxygen pump (pumping current) is adjusted such that theelectromotive force E will be maintained at a predetermined level, asubstantially linear relationship is obtained between the pumpingcurrent and the content of oxygen in the gas to be analyzed, therebyenabling the determination of the oxygen level of that gas.

When the air/fuel mixture is in the rich region, the oxygenconcentration electrochemical cell of the oxygen sensor put into theexhaust gas will produce an electromotive force even if the oxygen pumpis not actuated to create a differential oxygen partial pressure betweenthe opposing electrodes. Therefore, in order to maintain theelectromotive force from the oxygen concentration electrochemical cellat a constant value, the direction of the pumping current flowingthrough the oxygen pump should be reversed. More specifically, theoxygen at the electrode on the side of the oxygen concentrationelectrochemical cell facing the diffusion compartment is consumed byreaction with unburned hydrocarbons and carbon monoxide in the exhaustgas, and the differential oxygen partial pressure existing between theside of the cell facing the diffusion compartment and that of the cellwhich is in contact with the atmosphere is increased so much that theresulting electromotive force will exceed a predetermined level. As aresult, in order to maintain the electromotive force at thepredetermined value, oxygen must be pumped into the diffusioncompartment by operating the oxygen pump. To this end, the pumpingcurrent is caused to flow in the direction opposite to that used whenthe air/fuel mixture is in the lean region. In addition, the amount ofthe required pumping current is proportional to the amounts of unburnthydrocarbons and carbon monoxide in the exhaust gas. Therefore, thepumping current that is caused to flow in the rich region is alsoproportional to the A/F ratio of the air/fuel mixture.

To summarize the foregoing explanation, if the pumping current that iscaused to flow through the oxygen pump in the A/F ratio sensor of thepresent invention is adjusted so that the electromotive force generatedby the oxygen concentration electrochemical cell will be maintained at apredetermiend level, the resulting pump current will be proportional tothe A/F ratio of the air/fuel mixture being sensed. This linearrelationship is shown in FIG. 3.

The A/F ratio may also be determined from the electromotive force thatis attained when the pumping current is maintained at a constant level.The relationship explaining this possibility may be best understood byreference to FIG. 4, wherein the direction of pumping current is assumedto be positive when oxygen is pumped out of the diffusion compartment.

When the pumping current I_(p) is zero, the electromotive force makes anabrupt change at an A/F ratio that is substantially equal to thestoichiometric value (A/F=14.6). When the pumping current I_(p) isnegative (i.e., when oxygen is fed into the diffusion compartment), theelectromotive force will make an abrupt change in the rich region. Ifthe pumping current I_(p) is positive, the slope of the change in theelectromotive force is less steep than in the case of I_(p) =0 or I_(p)<0, but it still makes an abrupt change in the lean region. In otherwords, the point at which the electromotive force makes an abrupt changeshifts from the rich to lean region as the pump current I_(p) increasesfrom negative to positive values.

It is known that the A/F ration sensor exhibits better responsecharacteristics if the oxygen partial pressure in the diffusioncompartment is at the lower end. In accordance with the presentinvention, the diffusion compartment is in a flat form and helps toprovide an even better A/F sensor performance by forming a uniformdistribution in terms of the partial pressure of the gas to be analyzedwithin the diffusion compartment (i.e., the distribution of partialpressure as between the opposing electrodes on the oxygen concentrationelectrochemical cell and the oxygen pump).

One embodiment of the A/F ratio sensor of the present invention ishereunder described with reference to FIGS. 1 and 2, which are a partialcutaway perspective view and an exploded view of the sensor,respectively.

The sensor of the embodiment shown is constructed so that a diffusioncompartment 1 is formed between one oxygen concentration electrochemicalcell 2 and one oxygen pump 3 that are disposed in a face-to-facerelationship.

The oxygen concentration electrochemical cell 2 is composed of a solidelectrolyte plate 4 (7×45×0.6 mm) that is made of an Y₂ O₃ -ZrO₂ solidsolution and which has electrodes 5 and 6 formed on opposite sides ofthe plate by thick-film deposition of platinum containing 5 wt. % of anY₂ O₃ -ZrO₂ solid solution. The side of the solid electrolyte plate 4that is opposite the side facing the diffusion compartment 1 is providedwith a channel former 9 that is the combination of a U-shaped stressrelaxing layer 7 (thickness, 1.0 mm; outer dimensions, 7×45 mm; innerdimensions, 5×43 mm) made of a sintered mixture of Al₂ O₃ and ZrO₂ andan Al₂ O₃ support (7×45×0.8 mm). Atmospheric air is introduced to makecontact with the electrode 5 on the oxygen concentration electrochemicalcell 2 through a channel 10 provided by the channel former 9. A heatingelement 11 is formed on the side of the support 8 facing the channel 10.

The oxygen pump 3 is similar to the oxygen concentration electrochemicalcell 2 in that it is composed of a solid electrolyte plate 12,electrodes 12 and 13, and a channel former 17 comprised of a stressrelaxing layer 15 and a support 16. Atmospheric air is introduced tomake contact with the electrode 13 on the oxygen pump 3 through achannel 18 provided by the channel former 17. A heating element 19 isformed on the support 16.

The diffusion compartment 1 is formed by sandwiching a diffusioncompartment/diffusion limiting section former 20 between the solidelectrolyte plate 4 of the oxygen concentration electrochemical cell 2and the solid electrolyte plate 12 of the oxygen pump 3. The member 20is made of a sintered mixture of Al₂ O₃ and ZrO₂, and has a generallyU-shaped form (thickness, 0.1 mm; outer dimensions, 7×45 mm; innerdimensions, 3×9 mm) provided with a diffusion limiting section 30(0.1×0.1 mm in cross section) on three sides. The aperture of eachdiffusion limiting section may be filled with a porous material(typically made of bound alumina particles having a maximum porosity of3 microns or less and an average porosity of 1.2 microns) so as toprovide a greater diffusion: resistance. The porous material usedadditionally to provide an increased resistance to gas diffusion mayhave a comparatively high porosity. Such a highly porous material isreasonably insensitive to clogging by dust particles, and hence has goodresistance to deterioration of the performance of the sensor. Inaddition, the highly porous material is fairly easy to fabricate.

The diffusion compartment 1 in the A/F ratio sensor of the embodimentshown is in a flat form and the area of the electrodes on the oxygenpump 3 is sufficiently larger than the capacity of the diffusioncompartment 1 to permit rapid diffusion limitation and reduce thedifferential partial pressure of the gas to be analyzed existing betweenthe opposing electrodes on the oxygen concentration electrochemical celland the oxygen pump, thereby ensuring good response characteristics andminimizing the required pumping current.

The stress relaxing layers 7 and 15 that are used in the channel formers9 and 17, respectively, and each of which is made of a sintered mixtureof Al₂ O₃ and ZrO₂ are effective in preventing warpage of the A/F ratiosensor or its failure due to thermal expansion mismatch during service.It is to be noted that warpage of the sensor that may tend to occurduring use can be substantially eliminated because it has asubstantially symmetric configuration with respect to the plane ofdiffusion of compartment 1.

The heating elements 11 and 19 are advantageous in that they provideeasy temperature compensation.

FIGS. 3 and 4 show the operating characteristics of the A/F ratio sensorof the present invention. As was noted above, FIG. 3 shows the pumpingcurrent vs. A/F ratio for a constant output voltage being produced fromthe oxygen concentration electrochemical cell 2, and FIG. 4 shows therelationship between the A/F ratio and the output voltage produced fromthe oxygen concentration electrochemical cell 2 when a constant pumpingcurrent is applied.

In the embodiment shown, an air-introducing channel is provided suchthat the electrode on that side of the oxygen pump which is opposite theside facing the gas diffusion compartment is exposed to the atmosphere.If desired, said side may be merell exposed to the exhaust gas so thatoxygen may be picked up directly from the exhaust gas or indirectly fromoxygen-containing components in the exhaust.

In accordance with the present invention, the diffusion compartment isin a flat form and has a gas diffusion limiting section at three pointsof its periphery. This permits the use of large electrodes for theoxygen pump in comparison with the capacity of the diffusioncompartment. Any change in the partial pressure of the gas to beanalyzed in the atmosphere in the diffusion chamber can be quicklycancelled to provide a uniform distribution in terms of the partialpressure between the opposing electrodes, thereby enabling fabricationof an A/F ratio sensor having good response and high gas diffusionlimiting efficiency. Particularly good results are obtained when theapertures formed in the periphery of the diffusion compartment arefilled with a porous material.

The stress relaxing layer used in each of the channel formers has theadvantage of preventing warpage of the A/F ratio sensor during use, andfailure due to thermal expansion mismatch.

Since both of the oxygen concentration electrochemical cell and theoxygen pump are provided on a single solid electrolyte plate, the use ofsolid electrolytes is reduced to make contribution to the conservationof resources.

The operating range of the sensor can be expanded and its responseimproved by using a second oxygen pump.

In a preferred embodiment, a layer of particles may be placed in thediffusion compartment in a thickness equal to that of the latter; thisis effective in preventing deformation of the diffusion compartment infabrication procedures, especially in the firing step, thereby enablingmass production of acceptable sensors.

We claim:
 1. In a method of fabricating an air/fuel ratio sensor of thetype comprising an oxygen concentration electrochemical cell, an oxygenpump disposed in face-to-face relationship with said oxygenconcentration electrochemical cell to form a gap therebetween anddiffusion limiting means for enclosing a substantial portion of theperiphery of said gap except for at least one aperture to thereby form agas diffusion compartment which is surrounded by said diffusion limitingmeans and which communicates with a gas to be analyzed through saidaperture, said method comprising the steps of forming an unsinteredmultilayer structure of a first solid electrolyte plate for said oxygenconcentration electrochemical cell, a spacer element for said diffusionlimiting means and a second solid electrolyte plate for said oxygen pumpand thereafter sintering said multilayer structure to form said sensor,said method further comprising the step of disposing a plurality ofsupport members within said diffusion compartment prior to saidsintering step, each support member having a dimension substantially thesame as the width of said gap and being of a material which remains insaid gap and substantially retains said dimension after said sinteringstep, wherein said support members comprise granulated particles havinga diameter substantially equal to the width of said gap.
 2. A methodaccording to claim 1, wherein said disposing step comprises disposingsaid granulated particles in a single layer so that each particle issubstantially in contact with both sides of said gap.
 3. In a method forproducing an air/fuel ratio sensor having an oxygen concentrationelectrochemical cell and an oxygen pump, including the steps of forminga laminate including an unburnt solid electrolyte plate for said oxygenconcentration electrochemical cell, an unburnt spacer having a hollowspace therein, and an unburnt solid electrolyte plate for said oxygenpump, with a gap existing at said hollow space between said oxygenconcentration electrochemical cell and said oxygen pump, said gap havinga width of no more than 0.2 mm and no less than 0.01 mm, to form a gasdiffusion compartment communicating with a gas to be analyzed by a gasdiffusion limiting means, and thereafter sintering said laminate to formsaid sensor, said method further comprising the step of arranging asingle layer of ceramic particles, having substantially the samediameter as the width of said gap, in said gas diffusion compartmentprior to said sintering step, said ceramic particles remaining in saidgap and substantially retaining said diameter after said sintering stepto thereby prevent deformation of said diffusion compartment.